1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing composite electrical insulators comprised of a polymeric material.
2. Description of Related Art
There is known a composite electrical insulator which includes an elongate and solid or hollow core rod of fiber-reinforced plastics (FRP), a sheath covering the core rod over substantially the entire length thereof, and a plurality of sheds projecting radially outwards from the sheath and spaced from each other in a longitudinal direction of the core rod. The sheath and the sheds are integrally formed on the core rod by injection or transfer molding of appropriate polymeric material having an electrically insulating property, such as silicone rubber, ethylene propylene copolymer (EPM), ethylene propylene diene copolymer (EDPM), polyurethane, etc. The core rod has a longitudinal end portion provided with a seat which is integral with the sheath and to which a metal fitting can be fixedly secured. Such a composite insulator has been actually put into practical applications, particularly in any use environment which can draw out various functional advantages of the composite insulator.
In order to manufacture composite insulators, there is typically used a mold assembly which includes upper and lower mold halves defining a mold cavity when they are tightened and thereby closed. The mold cavity has an inner surface which substantially exactly corresponds to the outer shape of the sheath, sheds and seats to be formed on the core rod. Thus, with the core rod placed in the mold cavity, the polymeric material is injected or introduced into the mold cavity and cured therein to form the sheath, the sheds and the seats which are integral with the core rod. On such occasion, it is important not only to precisely position the core rod within the mold cavity, but also to prevent formation of burrs along the parting surface of the mold halves at locations corresponding to the seats at the end portions of the core rod, and to prevent leakage of the polymeric material from the longitudinal end portions of the mold assembly due to the following reasons.
That is to say, formation of burrs along the parting surface of the mold halves, hence along the seats at the end portions of the core rod, is disadvantageous in that the connection strength of the metal fitting which is fitted and crimped onto the seats at the end portions of the core rod becomes non-uniform in the circumferential direction of the core rod, and further in that rain water or like foreign matter are liable to intrude inside the metal fitting when the composite insulator is in use, thereby degrading the performance of the insulator. On the other hand, leakage of the polymeric material from the end portions of the mold assembly is disadvantageous in that it lowers the yield of material and contaminates the mold assembly.
In a conventional mold assembly, it has been difficult or practically impossible to arrange a seal device within the assembly, which serves to effectively prevent formation of burrs along the parting surface of the mold halves, and/or sufficiently prevent leakage of the polymeric material from the end portions of the mold assembly in tight contact with the core rod within the mold cavity. Moreover, it has been also difficult to provide a vent passage within the mold assembly, which serves to smoothly discharge gas from the mold cavity without leakage of the polymeric material therethrough.